Need help with my shoes.

I'm a beginner. I recently bought a pair of tennis shoes so I don't have to play in my pair of old running shoes. The pair I got ended up being about a half size too big. They're comfortable and great, but when I play my feet move around a little inside them, and I end up with blisters.

I can't return them because they were on clearance, and I'm on a budget and can't go get a different pair. Is there a good insert I can put in these ones so that they fit better?